There has been a recent trend in the detergents industry towards powders of high bulk density, prepared by processes that eliminate, or do not introduce, the porosity typical of traditional spray-dried powders. These include post-tower densification of spray-dried powders, and, more preferably, wholly non-tower routes involving dry-mixing, agglomeration, granulation and similar processes.
For example, EP 544 492A (Unilever) discloses high bulk density powders containing a high level of high performance surfactants (ethoxylated nonionic surfactant plus primary alcohol sulphate), zoolite builder, and other optional ingredients. The use of relatively high levels of zoolite allows the formulation of free-flowing powders containing high levels of these mobile surfactants.
These compositions consist essentially of a dense granular base containing surfactants, zeolite, sodium carbonate, soap and other minor ingredients, prepared preferably by a wholly non-tower mixing and granulation process, for example, in a high-speed mixer/granulator which combines high speed stirring and cutting actions.
To the base powder are admixed (postdosed) further ingredients which may be unsuitable for incorporation in the base powder for various reasons, for example, bleaching persalts, bleach precursors and bleach stabilisers, enzyme granules, foam control granules, and perfume.
With formulations of this type, some problems have been experienced in the delivery of the active ingredients of the powder to the wash in an automatic washing machine. Delivery is a two-step process: the first step is the dispensing of the powder into the wash liquor, either from the dispenser drawer of the washing machine or from a dispensing device (a wash ball or similar) supplied by the powder manufacturer; and the second is dissolution of the powder once it arrives in the wash water.
It has surprisingly been found that in high bulk density powder of the type mentioned above, delivery is improved by incorporating a citric acid salt of small particle size in the dense granular base powder. If desired, additional citrate (not necessarily of same particle size) may be postdosed.
Citrates are well known as detergency builders used to supplement zeolites. Their use in zeolite-built powders is disclosed, for example, in EP 313 143A, EP 313 144A, EP 448 297A and EP 448 298A (Unilever); GB 1 408 678, EP 1310A, EP 1853B, EP 326 208A, EP 456 315A and WO 91 15566A (Procter & Gamble); DE 2 336 182C (Lion); and GB 2 095 274B (Colgate). The art discloses the incorporation of sodium citrate in conventional porous spray-dried base powders, and also discloses the postdosing of sodium citrate.
High bulk density detergent powders containing sodium citrate are disclosed in our copending International Patent Application No. PCT/EP94/01291 filed on 26 Apr. 1994, but the sodium citrate is postdosed as a relatively coarse material (typical average particle size above 800 .mu.m).
EP 425 277A (Unilever) discloses detergent powders of high bulk density prepared by densifying a spray-dried base powder. The powders contain soap, nonionic surfactant, zeolite and sodium citrate.
EP 349 201A (Procter & Gamble) describes the preparation of a compact detergent powder by a process in which an aqueous surfactant paste is mixed with dry detergent builders to form a dough, and the dough is then chilled and granulated by fine dispersion mixing to form particles. Compositions containing zeolite and high levels of sodium citrate (typically 17-27 wt %) are disclosed.
The incorporation of citrate of defined particle size in a non-spray-dried base to improve the delivery and dissolution of a high bulk density detergent powder has not been described in the literature.